Process of producing corrosion resistant coatings on magnesium and high percentage magnesium alloys



Patented Nov. '1, 1938 UNITED STATES SISTANT COATINGS HIGH PERCENTAGE Josef Martin Michel,

signor, by mesne assi Development Corporation,

Delaware No Drawing. Application August 21, 1937, Se-

rial No. 160,272. In 1936 5 Claims.

The present invention relates to a process of producing corrosion resistant coatings on mag nesium and high percentage magnesium-alloys.

It has been suggested that a treatment of magnesium and high percentage magnesium alloys with aqueous or acid solutions containing chromic acid or chromates of the alkali metals leads to the formation of a coloured protective layer on the metal which increases its resistance to corroding influences. For the purpose of imparting definite colour shades to the metal surface, an addition of heavy metal salts to the said solutions has also been suggested. In practice, however, only dichromate solutions have been hitherto used for the aforesaid purposes since experience shows that with chromic acid solutions of ordinary concentration, particularly such as do not simultaneously contain heavy metal salts, there is practically no coating formed.

On the'other hand, the treatment of high percentage magnesium alloys with acid dichromate solutions is attended with a very heavy consumption of dichromate which in turn results in rapid alteration of the acidity and the dichromate concentration of the bath. Moreover, when using acidulated baths, a; comparatively strong attack of' the-metal surface to be treated takes place whilst, when using, as is the custom, solutions in which acidulation is brought about by nitric acid, also nitrous gases are formed.

The process according to tire present invention avoids these drawbacks while at the same time affording the possibility of forming coatings having a wide range of colour shades, and also of imparting to the base metal an improved resistance to corrosion. In accordance with the present invention magnesium and high percentage magnesium alloys are treated with an aqueous solution ofbetween about 0.1 and l per-cent chromic acid, with the exclusion of other saline additions, but in the presence of chlorides and/or sulphates in the quantities usual in ordinary tap water. It has been ascertained that only when using chromic acid in the aforesaid proportions and dissolved in water having the usual chloride and/or sulphate content of tap water, coatings are formed, and that such coatings are uniformly well adherent on the base metal. Preferably the content of chromic acid in the solution should not exceed about 4 grms. per litre.

In contradistinction to the normal type ,of dipping baths employed for magnesium and high percentage magnesium alloys and consisting of nitric acid and sodium dichromate (e. g. 500 cm Bitterfeld, Germany, as-

PATENT OFFICE ON MAGNESIUM AND MAGNESIUM ALLOYS gnments, to Magnesium a corporation of Germany September 4,

of 40% HNOs, 500 cm H20, grms. K2Cr2O1-- solution No. I) a 0.4% chromic acid solution (4 grms. CrOa per litre-solution No. 11) causes practically no attack on the metal to be treated, as may be seen from the following Table I. The practically negligible increase in weight, of the metal, when using chromic acid solution, is caused by the formation of the protective layer on the surface of the metal.

The improvement obtained by the use of baths in accordance with the present invention in respect of resistance to corrosion by water may be seen from the following Table II.

Table II Decrease in weight in grams per square metre surtaoe alter 48 hours alloy Solution I 'Solution 11 1% Zn, 6% Al, 0.5% Mn, balance Mg- 7 b i l )5 Mn, 0.3 Si alance M 4.

The saving effected in consumption of C103 by the process according to the present invention over the former treatment is best shown by the fact that per square metre of surface treated and with a duration of immersion'of 30 seconds in bath No. I (former process) 500 grms. HNO: (40%) and 3 grms. .KZCIZO'! were consumed, whereas with bath No. II and a duration of immersion of four minutes the consumption is only about 1.1 grms. CrOs.

The colour shade of the coating produced on the metal surface may be varied by varying the temperature or the bath according to the invention. With a temperature of 20 C. the shade is similar to that oi brass, whereas with higher temperatures the shade becomes darker. It is of some importance, however, to note that with temperatures above normal the duration of immersion necessary for producing a uniformly adherent sound coating is reduced, being approximately four minutes for a temperature of 20 and falling oil to one and a half minutes at 100 C.- The adherence and also the soundness of the coatings are not reduced when working with higher temperatures, provided that the duration of immersion is correspondingly shortened.

1. Process of producing corrosion resistant coatings on magnesium and high percentage magnesium alloys by a treatment with a solution of between about 0.1 and about 1 percent of chromic acid in water, and containing an ion selected from the group consisting oi chloride and sulphate ions in an amount approximately equal to the content 01 such ion in ordinary tap water.

2. Process of producing corrosion resistant coatings on magnesium and high percentage magnesium alloys by a treatment with a solution of 0.4 percent of chromicacid in water, and containing. an ion selected from the group consisting of chloride and sulphate ions in an amount approximately equal to the content 01' such ion in ordinary tap water.

3. Process of producing corrosion resistant coatings on magnesium. and high percentage magnesium alloys by a treatment at elevated temperatures with a solution or between about 0.1 and'about 1 percent of chromic acid in water, and containing an ion selected from the group consisting of chloride and sulphate ions in an amount approximately equal to the content of such ion in ordinary tap water.

4. Process of producing corrosion resistant coatings on magnesium and high percentage magnesium alloys by treatment with an aqueous solution. containing between about 0.1 and about 1 per cent 01' chromic acid and having a chloride ion content approximately equal to that of. ordinary tap water. I

5. Process of producing corrosion resistant coatings on magnesium and high percentage magnesium alloys by treatment with an aqueous solution containing between about 0.1 and about 1 per cent of ,chromic acid and having'a sulphate ion content approximately equal to that oi ordinary tap water.

' JOBEF MARTIN MICHEL. 

